In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Today, coated cemented carbide inserts with a binder phase-enriched surface zone are commonly used for machining of steel and stainless steel materials. The binder phase enriched surface zone widens the application area towards tougher cutting operations.
It has been known for some time how to produce binder phase enriched surface zones on cemented carbides containing WC, binder phase and cubic carbide phase, e.g.—see Tobioka (U.S. Pat. No. 4,277,283), Nemeth (U.S. Pat. No. 4,610,931) and Yohe (U.S. Pat. No. 4,548,786).
The patents by Tobioka, Nemeth and Yohe describe methods to accomplish binder phase enrichment in the surface region by dissolution of the cubic carbide phase close to the insert surfaces. Their methods require that the cubic carbide phase contains some nitrogen, since dissolution of cubic carbide phase at the sintering temperature requires a partial pressure of nitrogen, and nitrogen activity within the body being sintered exceeding the partial pressure of nitrogen within the sintering atmosphere. The nitrogen can be added through the furnace atmosphere during the sintering cycle and/or directly through the powder. The dissolution of cubic carbide phase, preferentially in the surface region, results in small volumes that will be filled with binder phase giving the desired binder phase enrichment. As a result, a surface zone consisting of essentially WC and binder phase is obtained. Although the cubic carbide phase is essentially a carbonitride phase, the material is herein referred to as a cemented carbide.
EP-A-1 026 271 relates to a coated cemented carbide insert with a highly alloyed Co binder phase. The insert has a binder phase-enriched surface zone of a thickness of <20 μm and along a line in the direction from the edge to the centre of the insert the binder phase content increases essentially continuously until it reaches the bulk composition. The use of a thin binder phase enriched surface zone is said to be favourable to avoid plastic deformation.
The new coating design for Ti(C,N)—Al2O3—Ti(C,N) coatings is disclosed in U.S. Pat. No. 6,221,469 and has been found to perform well and outperform competing products based on the layer design where Al2O3 is the outermost layer, covered only with a thin layer of TiN. This kind of new layer design can be applied on conventional or surface-modified cemented carbides according to U.S. Pat. No. 6,221,469. We have also shown that toughness of this coating can further be increased if the Al2O3 layer consists of α-Al2O3 (U.S. patent application Ser. No. 10/323,905). The α-Al2O3 intermediate layer is especially important when the tool is used in interrupted cutting or when cooling is applied, i.e.—in applications where the heat flow into the substrate is lower.
The thick Ti(C,N) coating is important because in many common steels both α-Al2O3 and κ-Al2O3 are clearly outperformed by MTCVD Ti(C,N) as far as flank wear is concerned. Consequently, in order to reduce flank wear it is important to protect the Al2O3 layer by a layer of Ti(C,N), for instance, according to U.S. Pat. No. 6,221,469. The Ti(C,N) layer is preferably deposited by MTCVD on α-Al2O3.